A liquid level measuring technique using ultrasonic waves involves: placing ultrasonic sensors on an outer surface of a container that stores a liquid; transmitting ultrasonic waves from the ultrasonic sensors; and utilizing a phenomenon in which an intensity attenuation rate of each echo wave that is received after being reflected on an inner surface of the container is different depending on whether a reflection point of the echo wave is in contact with the liquid or a gas.
That is, an ultrasonic wave reflected on an interface between the gas and the container hardly attenuates, whereas an ultrasonic wave reflected on an interface between the liquid and the container observably attenuates.
Then, a liquid level is measured by recognizing which of the gas and the liquid a placement position of each of the plurality of ultrasonic sensors along a vertical direction of the inner surface of the container is adjacent to (for example, Patent Documents 1 and 2).